Screw compressors will usually comprise two helical compressor rotors which closely mesh within a compressor housing. There is a female and a male rotor, which are mounted with the spirals of their respective helices orientated in opposite directions. The rotors are driven and as they rotate, fluid is trapped in the space between the rotors and the housing. This enables the fluid to be compressed by the action of the rotors.
The casings of such screw compressors generally comprise three separate sections: a main central housing, an inlet housing and an outlet housing. In some case, two of these components may be combined into one contiguous piece, such as the inlet casing and the main casing. All three of these are structural elements i.e. they bear load transmitted through bearings.
The inlet and outlet housings will have mounts for the rotor shafts, with the main bodies of the rotors mounted within conjoined rotor chambers. Since the rotors may rotate at high speed and require precise tolerances to enable correct operation, thrust and radial bearings are used to maintain correct relative positioning.
Compressing the gas creates a pressure load that is borne by the rotors.
Compressing fluids creates heat and this heat causes expansion of the materials used to manufacture the housings and rotors. Since they may be formed from different materials, are different shapes and are exposed to varying temperatures, the rotors and the casings will not expand uniformly, and hence a thermal load is created by the rotors expanding longitudinally against the mounts.
The three housings are connected to one another usually be arrangements of flanges and bolts, and the loads acting upon the system must be resisted by these arrangements.